1. Field of the Invention
The present invention relates to a receiving apparatus, a receiving method, and a radio communications system for demodulating (separating) a signal using two or more antennas.
2. Description of the Related Art
In the fourth generation mobile communications, a radio communications method that is capable of providing a high transmission speed is required. From this viewpoint, a MIMO (multiple inputs and multiple outputs, i.e., two or more I/O) multiplexing method attracts attention, wherein each of antennas (for transmission and reception) transmits a different signal from others at the same time and the same frequency using MIMO channels.
The MIMO multiplexing method is described with reference to FIG. 14 that shows the configuration of a MIMO communication system using two or more antennas. According to this system, the transmission speed is increased in proportion to the number of transmitting antennas without increasing a transmission bandwidth by antennas 10111 through 1011N transmitting respective different signals at the same frequency, and antennas 10211 through 1021N simultaneously receiving all the different signals (for example, Non-patent Reference 1).
Further, BLAST (Bell Labs Layered Space-Time) is one of the technologies generically named MIMO. According to BLAST, different signals are simultaneously transmitted at the same frequency, i.e., a parallel transmission, from two or more transmitting antennas; and, on the receiving side, the signals are separated by diversity reception that is controlled by interference suppression and by a replicas subtraction (for example, Non-patent Reference 1).
As described above, while it is possible to realize a high transmission speed by the MIMO multiplexing method, since different data sequences are transmitted from the transmitting antennas at the same frequency and at the same time slot, a receiving unit is required to separate the signals in order to extract the data sequences transmitted from each transmitting antenna from the received signal for demodulation.
Various methods are proposed for separating the MIMO multiplexed signals. For example, according to a signal separation algorithm using a linear filter such a minimum mean square error (MMSE) method, and a zero forcing (ZF) method, signals received by antennas, the number of which antennas is equal to or greater than the number of the transmitting antennas, are compounded such that receiving power from transmitting antennas other than a target transmitting antenna is suppressed (minimized). According to this method, the amount of operations on the receiving side is relatively small.
Further, according to a signal separation algorithm using a maximum likelihood detection method (MLD), replica candidates of signals received from each transmitting antenna are generated, and a replica of a received signal that gives the smallest Euclidean distance between the received signal and a sum of the replica candidates of the signals received from all the transmitting antennas is obtained. In this way, the MLD method gives the most probable signal sequence of each transmitting antenna. The MLD method provides more accurate signal separation and superior demodulation performance as compared with the MMSE method; however, the amount of operations required of signal separation exponentially increases as the number of the antennas is increased. Then, a proposal (for example, Non-patent Reference 2) is made wherein the amount of operations is decreased in the MLD by greatly decreasing the number of signal point candidates for calculating the square Euclidean distance using QR factorization.
By the way, conventional direct spreading (DS) CDMA is a communication method wherein a secondary modulation is performed for spectrum-spreading a signal that is modulated by conventional information data with a high-speed spreading signal such that two or more parties can communicate using the same frequency band. In the radio communications using DS-CDMA, as the bandwidth used for signal transmission becomes great, multipath fading (frequency selective fading) is generated, and a transmission signal is received through two or more paths (multipath) having different propagation delay times.
While receiving quality can be improved in DS-CDMA with a rake reception method, wherein two or more signals through the multipath are compounded, interference (multipath interference) is generated between different paths. For this reason, the receiving quality improvement by the rake reception method is offset by the multipath interference.
The magnitude of the multipath interference is proportional to the inverse number of a spreading rate that is defined by a ratio of a chip rate (representing a speed of multiplication by the spreading signal) to a symbol rate of information symbols. For this reason, if the spreading rate is brought close to 1 in an attempt to increase an information bit rate, degradation of the receiving quality due to the multipath interference becomes dominant rather than the improvement obtained by the rake reception method. This poses a problem in that the receiving quality is degraded when transmitting at a high speed. Then, in an attempt to cope with the problem, a multipath interference canceller is proposed (for example, Non-patent Reference 3).
According to the multipath interference canceller as disclosed by Non-patent Reference 3, high quality reception under a multipath environment is realized by
estimating a received signal sequence for every path based on an signal sequence that is estimated by a provisional rake receiving result and a channel coefficient (complex envelope of a propagation path) of each reception path,
deducting all estimated signal sequences obtained from paths other than a certain path (a target path) from the received signal, and
repeating the deducting operation for all the paths. In this way, the multipath interference of the signal of every path is reduced. Then, using the signals, for the multipath interference of which signals have been reduced, a final rake reception is carried out.
Further, 2-dimensional MMSE is proposed as a signal separating method, whereby the multipath interference in MIMO multiplexing in the radio communications using DS-CDMA is reduced, With the 2-dimensional MMSE, both interference generated by received signals from other transmitting antennas and multipath interference are simultaneously reduced.    [Non-patent Reference 1] G. J. Foschini, Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multiple Antennas, Bell Labs Technical Journal, Vol. 1, No. 2, autumn 1996, pp 41 through 59.    [Non-patent Reference 2] Bin Dong, Xaodong Wang, and Amaud Doucet, “Sampling-based Near-optimal MIMO demodulation Algorithms” in Proc. 42nd IEEE Conference on Decision and Control, Hawaii, December, 2003    [Non-patent Reference 3] K. Higuchi, A. Fujiwara, and M. Sawahashi, “Multipath Interference Canceller for High-Speed Packet Transmission With Adaptive Modulation and Coding Scheme in W-CDMA Forward Link,” IEEE J. Select. Areas Commun., Vol. 20, No. 2, pp. 419 through 432, February 2002.    [Non-patent Reference 4] Frederik Petre et. al, “Combined Space-Time Chip Equalization And Parallel Interference Cancellation For DS-CDMA Downlink With Spatial Multiplexing” in Proc. IEEE PIMRC 2002.